1. Field of the Invention
The invention relates to a method and a device for efficient usage of the cooling air leakage through the gaps of the combustor liner segments to get an acoustic damping of combustion pulsations.
2. Discussion of Background
In modern industrial gas turbines operating with premixed combustion flames, the suppression of pressure pulsations is an important task related to the quality of the combustion process and to the structural integrity of engines. In order to reduce pulsations acoustic damping techniques may be employed.
In conventional premixing swirl burners the flame is stabilized by means of the vortex break-down bubble produced by swirling the premixed mixture (see for instance the dual burner as described in EP 0210462 or EP 0321809). In this type of combustion systems pulsations caused by the combustion process may be damped using axoustic screens. Acoustic screens are widely used to dampen noise generated in rocket engines or in aircraft engines. An acoustic screen consists of a perforated screen lining the engine ducts (e.g. the fan duct of a turbofan engine). The perforated screen has a back imperforated screen and in the cavity between the two screens a honeycomb core sandwiched may be located. The goal of the acoustic screen is to realize a wall not fully reflecting on the acoustic point of view and able to dampen pulsations in a broad range of frequency. The acoustic behaviour of the acoustic screen is defined by means of its impedance Z=R+iX, i.e. the ratio between acoustic pressure and velocity normal to the wall both defined in the frequency domain. The real part R of the impedance is the resistance and the imaginary part X is the reactance. The acoustic screen resistance R is related to dissipative processes occuring in the acoustic screen holes. The main dissipative effect is in the conversion of acoustic energy into shedding of vorticity generated at the rims of the screen holes, convected downstream and finally dissipated into heat by turbulence. The acoustic screen reactance X represents the inertia of the fluid fluctuating in the holes and in the back cavity under the effect of the acoustic field. To dampen specific acoustic modes, acoustic screens are designed to have R close to ρc (being ρ the fluid density and c the fluid speed of sound) and X<<ρc in the range of frequency where such modes occur. Note that the conditions R=ρc and X=0 correspond to the anechoic condition, i.e. the full absorption of the acoustic energy of a normally incident plane wave. The efficiency of an acoustic screen is strongly related to the portion of surface the acoustic screen covers. Different acoustic screen designs have been proposed, where the band of damped frequency was eventually extended by means of multi-layer acoustic screens (U.S. Pat. No. 3,439,774; U.S. Pat. No. 3,640,357; U.S. Pat. No. 5,782,082) or by a non-uniform distribution of honeycomb cells (U.S. Pat. No. 3,831,710).
In combustion chambers, used in large industrial gas turbines, but also found in other combustion engines, a solid casing forms the outer structure of the combustion chamber. For cost reasons, this relative large part is usually made of a material like cast steel, which cannot withstand the hot combustion air and thus has to be protected. This is done by covering the inner surface of the combustion chamber casing with a liner or shell made of a high temperature resistant material (Ni base alloy or even structural ceramics) which is cooled from the back side, to keep the high temperatures away from the casing material. From a design point of view, it is advantageous to use multiple segments for the inner liner instead of one single piece, because assembly is facilitated and the thermal stresses in the liner are kept lower. A disadvantage of the segmented liner is that there are always gaps of finite width between the individual segments. As a consequence, there is a risk of hot combustion gas penetrating into the gaps, which in long term would damage the casing structure. Therefore, the gaps have to be purged by a controlled leakage of cooling air. As this additional cooling air is bypassed from the combustion process, it creates undesired performance and eficiency losses.